A technique of estimating a direction of a face from an image representing the face has been studied.
A face direction detection sensor for detecting the direction of, for example, a driver's face is known. This face direction detection sensor detects a face border and a face part based on edges detected from an input image, and estimates the center of the face based on the detected face border and face part. This face direction detection sensor then computes the face direction angle from the face border and the center of the face center, based on the hypothesis that the face is cylindrical.
In another technique of face direction estimation, positions of nodes that respectively correspond to feature points of a face are defined on a three-dimensional model, and the nodes are projected onto an image of the face. Then, an estimated error amount indicating positional displacement between the current three-dimensional model and the feature points is calculated based on projected points of the nodes, three-dimensional positions of the feature points of the face in the image are estimated based on the estimated error amount and the current three-dimensional model, and based on the result of the estimation, the face direction is estimated. Further, a technique is known in which the direction of a face on an image is converted to create a frontal face, and the face direction is estimated based on the degree of left-right symmetry of the frontal face (for example, see HAYASHI, et al., “Image Processing Technique for Monitoring Driver”, The Journal of the Institute of Image Information and Television Engineers, Vol. 61, No. 12, pp. 1701-1704, 2007, Japanese Laid-open Patent Publication No. 2007-249280, and Japanese Laid-open Patent Publication No. 2004-94491).